The present invention relates to a sample introduction methodology by means of which a volume of a liquid sample zone, which is to be introduced into an unsegmented liquid carrier stream, can be defined on the basis of a controlled combination of hydrostatic and hydrodynamic forces, said well-defined sample zone subsequently being transported to a flow-through analyzer in which a species present in the sample solution--possibly formed as a result of one or more chemical reactions taking place--can be evaluated quantitatively in a flow-through detector arrangement.
Assay by means of the Flow Injection Analysis (FIA) technique (see for example our U.S. Pat. Nos. 4,022,575 and 4,224,033) requires that a sample solution to be analyzed is introduced into an unsegmented carrier stream as a well-defined sample zone, the volume and geometry of which is strictly reproducible. The conventional sample introduction techniques can be divided into three groups which are based on the following principles:
(1) Direct injection of a precisely metered amount of sample solution into a carrier stream (see for example our U.S. Pat. No. 4,022,575);
(2) Insertion of a precisely metered amount of sample solution by means of a valve (see for example our U.S. Pat. No. 4,224,033); and
(3) Intercalation of a precisely metered amount of sample solution by means of a system of magnetic valves (see for example our U.S. Pat. No. 4,177,677).
By the first-mentioned method of sample introduction, the sample solution is injected by means of a syringe provided with a hypodermic needle which is pierced through the wall of the conduit in which the carrier stream is propelled, but this method of introduction is however not always sufficiently reproducible, nor does it lend itself readily to automation. By the introduction method referred to in Group (2), the use of sliding or rotary valves with exact bores of precisely metered volume (or possibly also provided with external sample loops in order to accommodate larger sample volumes)--and where the sample bores partly can be brought in a position where they, with sample solution, are made to be part of the carrier solution circuit so that the samples can be brought to be transported by the carrier stream--is most common as it yields highly reproducible results and is easily automated. Its drawbacks are the high cost of the valves which must be very precisely machined, and the mechanical wear of the moving parts which must be kept leakproof even after thousands of injections. By the introduction method referred to in Group (3), use is made of several (at least four) magnetic valves which, in a certain sequence and at predetermined time intervals, can be opened and closed. Also this method of introduction lends itself readily to automation, but it requires obviously ancillary electronic timing circuitry. Its greatest drawback is however that the elastic components on which the magnetic open/close valves mechanically operate eventually become worn and deformed as a result of the repeated localized pressure exerted by the wedge of the magnetic valves, and thus with time might fail to open or close completely, which will result in a slowly increasing malfunction which can be extremely difficult to identify.
The common denominator of all three sample introduction designs mentioned above is that the metered volume of sample solution to be introduced is defined by the volume of a solid container (bore, loop length, etc.) which immediately prior to introduction of the metered sample zone into the carrier stream is hermetically closed (that is, by introduction method 1 the metered volume of liquid sample corresponds to the volume trapped under the plunger of the hypodermic syringe; by introduction method 2 the liquid in the closed container is represented by that volume of sample liquid which is enclosed within the bore or sample loop of a sliding or rotary valve while the valve is being switched from the sampling to the introduction position; and by introduction method 3 the closed container, and the metered volume therein, corresponds to that volume which the magnetic valves entrap in a tube or conduit of a given length and diameter).
A problem which all three of these sample introduction systems have is that the closed container or conduit in which the well defined sample volume is placed or entrapped must continually be switched into and out of the carrier stream circuit or conduit during sample injection and sample loading. When the sample solution is being loaded into the closed sample container or conduit, the sample container must be disconnected or otherwise separated from the carrier flow stream. When the sample is to be introduced or injected into the carrier stream, the sample container must be switched into the system or otherwise connected so that the carrier stream sweeps through the container to transfer the sample solution into the carrier solution circuit or otherwise allows for the sample solution to be transferred from the sample container to the carrier stream circuit. This type of sample introduction or injection technique requires the use of numerous mechanical devices such as the valves and syringes discussed above which each have their own inherent drawbacks.